1. Field of the Invention
The present invention relates to an apparatus and method for calibrating focal plane arrays. The invention has particular relevance to infrared focal plane detectors.
2. Discussion of Prior Art
Conventional focal plane detectors typically comprise an array of optoelectronic sensor elements arranged to detect electromagnetic radiation incident on the sensors within the array. Generally, the optoelectronic sensors provide an output (for example a voltage, a current, a resistance, a capacitance or a charge) which varies as a function of the intensity of the electromagnetic radiation incident on the sensors. When used in conjunction with focusing means, for example a lens, the sensor outputs from such a focal plane array may be used to reconstruct an image of objects within a scene. The output from each sensor element within the array generally corresponds to a picture element (pixel) within such an image.
Sensor elements within a focal plane array are not identical, and as a result individual sensors within the focal plane array exhibit different responsivities to electromagnetic radiation incident on the array. The variations in responsivity between individual sensor elements generate undesirable noise (generally known as fixed pattern noise) in the output from the array which has to be calibrated out.
Any DC offset in the output from the sensor elements can be corrected for by presenting a bland defocused image to the focal plane array and mapping the response of each sensor element in the array. The slope of the response (linearity) of the sensor elements may also be measured by changing the intensity of the electromagnetic radiation incident on the array and re-measuring the response of each sensor element in the array. This is achievable by using a second bland defocused image, the second image being arranged to illuminate the array with a different intensity of electromagnetic radiation to the first image.
With this information, a focal plane detector array can be fully calibrated, and a clean image produced.
However, as the array ages, and as the intensity of electromagnetic radiation incident on the array varies (as a function of operating environment and with different scenes), it is possible that the array will need to be re-calibrated periodically.
Variable optics may be used to re-calibrate the array using the foregoing method of presenting bland defocused images to the array and characterising the response therefrom. However, variable optics increase the complexity and hence the cost of the focal plane detector.
Furthermore, this method is not optimised for re-calibrating infrared focal plane detectors (for example thermal imagers) due to a lack of bland defocused images having different temperatures. The accuracy of any adjustments to the linearity of the array will be dependent on the temperature separation of the first and second bland defocused images. Accordingly, a variable infrared source may be required as part of the re-calibration process.
Periodic re-calibration using flags or shutters placed into the optical path of the array are known. For example, in amorphous silicon cameras re-calibration occurs approximately once every minute or so (such devices are not very linear). These cameras insert a flag into the optical path to allow measurement of pixel variations and make a correction on that basis.
However, such devices increase the complexity and hence the cost of the focal plane detector. Furthermore, the shutter obscures the image from the scene during the re-calibration operation and this may be a nuisance if the re-calibration occurs automatically.
It is an object of the present invention to mitigate at least some of the disadvantages of the prior art. It is a further object of the present invention to provide an apparatus and method for re-calibrating periodically a focal plane array.